This invention relates generally to comparator circuits and, more particularly, to a comparator circuit in which the input signal voltage may vary from a first voltage substantially equal to a first supply voltage to a second voltage substantially equal to a second lower supply voltage.
Comparator circuits are known in which the input voltage levels are permitted to swing to one or the other supply voltages, but not both. For example, the LM124 manufactured by National Semiconductor permits the input voltage to go to ground. If, however, the input voltage were to reach a level equivalent to the positive supply voltage (e.g. 5 volts), the PNP current source which supplies a differential pair is turned off. In contrast to this, the .mu.A741 manufactured by Fairchild permits the input voltage to become as high as the positive supply voltage. If, however, the input voltage were to drop to ground, the current mirror incorporated into the circuit would turn off.